The favorable nuclear properties of 57Fe facilitate the study of heme iron by Mossbauer spectroscopy. Using and extending methods of measurement and analysis which have been developed over the past decade, this techniqe will be applied in an effort to understand the nature and mode of action of the active centers in hemoproteins. The focus of attention will be the mechanism of reversible oxygenation of hemoglobin and myoglobin, and an attempt to understand it in terms of the electronic wave functions which describe the active center. To contribute to this end, and because they are of interest in themselves, the investigation will deal with the mechanism(s) of cooperativity, the effect of substances such as organic phosphates which modify oxygen uptake, abnormal hemoglobins, artificial hemoproteins, species dependence, etc. Where possible hemoglobin in whole red cells will be compared with the purified protein. Comparative studies of different heme proteins as well as non-physiological compounds will be pursued to the extent that they support the primary goal. The main experimental technique will be observation of the Mossbauer spectra of heme iron in appropriate chemical conditions and as a function of temperature and magnetic field. Techniques for the reduction of data in terms of the spin Hamiltonian formalism are already fairly well in hand. The further reduction in terms of electron energy level schemes will be a central theoretical concern of the proposed investigation. An important aspect will be the study of model compounds because of their relative conceptual simplicty and because they lend themselves more readily to structural modification. Both the heme model compound work and the heme protein work will be carried out in collaboration with established workers in the chemical and biochemical fields.